Straight wings are easy. Wings that jut out from the fuselage at 90 degrees are simple to build. They’re also good: when the wings are perpendicular to on-coming air they get the most bite in air and generate the most lift. And yet, no jet-powered aircraft has straight wings.
Why? The Sound Barrier. (Technically, compressibility.) We always knew that to fly faster, you’d have to put in more power. And there’s even an equation that your tenth-grade algebra teacher used as an example of a polynomial equation, because he was really stretching for examples of polynomial equations. But near the speed of sound, going faster gets harder than your calculator would predict. Air stops being an ideal fluid and is just a bunch of molecules that can’t get out of the way fast enough.
What do you do when the math breaks down and going fast takes more power than expected? Aeronautics found an answer sure to warm your eleventh-grade trigonometry teacher’s heart: More Math! You “sweep” the wings, by turning them backwards. The plane and the wing face in different directions. Air now hits the wing off-angle, which means that when the plane is going at the speed of sound, the wing sees air hitting it at the speed of sound times the cosine of the sweep angle. Or in people-speak: less than the speed of sound!
450 mph is a dividing line. If a plane wants to fly faster, it needs swept wings. Below that sweeping the wings is a waste (it makes the wing heavier for no benefit). Jetliners have a V-shape for a different reason than flocks of geese have a V-shape (unless someone’s been dumping cocaine at your local park).
Sunday, January 29, 2012
Saturday, January 21, 2012
Pressurized Cabins
Flying higher is better. The air is less turbulent (most turbulence is caused by the sun’s warming the ground which warms low-level air which creates currents and bumps and queasiness). The view is nicer. And jet engines work more efficiently at higher altitudes. Jetliners were going to fly higher, above 30,000 feet compared to the 10,000 feet that earlier planes could fly.
One problem: much above 10,000 feet, humans start to pass out. Personally, I think 12 hour flights to Sydney in economy seats would be a lot easier if you just passed out 10 minutes after take-off. But the FAA thinks consciousness is safer, and when else is it societally acceptable to gorge yourself on 5 RomComs in a row? Earlier planes that had to fly high used oxygen masks. (Higher is also colder: at around 27,000 feet outside air hits -40 degrees Fahrenheit. This is why Bomber Jackets are leather and wool.) Obviously, paying passengers wouldn’t want to wear masks and insulation.
The answer: Pressurization.The engines’ turbochargers already compress air for the engine, so we divert some of this compressed air inside the plane. As the plane climbs, the air outside grows thinner, but the air inside the plane stays thick. Well, sort of thick: most planes are pressurized to about 8,000 feet. This is still thinner than we’re used to, so you probably won’t be running any marathons (also, you’d beat the screaming baby for the most-annoying-passenger prize).
This requires a new kind of plane. The skin is now effectively a balloon, holding air inside even as its higher pressure makes it want to escape. A tiny hole anywhere in the skin makes everyone in the plane pass out (surprise!) in the best case. In the worst case, it can be as disastrous as popping a balloon. Engineers deal with this in each plane, adding another complexity that they have to make fully safe while keeping us needy passengers warm, comfortable, and conscious.
[Pedants will point out the B-29 had pressurized cabins. Yes, it did, but the whole plane wasn’t pressurized yadda yadda technicalities.]
One problem: much above 10,000 feet, humans start to pass out. Personally, I think 12 hour flights to Sydney in economy seats would be a lot easier if you just passed out 10 minutes after take-off. But the FAA thinks consciousness is safer, and when else is it societally acceptable to gorge yourself on 5 RomComs in a row? Earlier planes that had to fly high used oxygen masks. (Higher is also colder: at around 27,000 feet outside air hits -40 degrees Fahrenheit. This is why Bomber Jackets are leather and wool.) Obviously, paying passengers wouldn’t want to wear masks and insulation.
The answer: Pressurization.The engines’ turbochargers already compress air for the engine, so we divert some of this compressed air inside the plane. As the plane climbs, the air outside grows thinner, but the air inside the plane stays thick. Well, sort of thick: most planes are pressurized to about 8,000 feet. This is still thinner than we’re used to, so you probably won’t be running any marathons (also, you’d beat the screaming baby for the most-annoying-passenger prize).
This requires a new kind of plane. The skin is now effectively a balloon, holding air inside even as its higher pressure makes it want to escape. A tiny hole anywhere in the skin makes everyone in the plane pass out (surprise!) in the best case. In the worst case, it can be as disastrous as popping a balloon. Engineers deal with this in each plane, adding another complexity that they have to make fully safe while keeping us needy passengers warm, comfortable, and conscious.
[Pedants will point out the B-29 had pressurized cabins. Yes, it did, but the whole plane wasn’t pressurized yadda yadda technicalities.]
Friday, January 6, 2012
Podded Engines
Every new technology has its quirks. Especially when that technology includes spinning pieces of metal at 1000 miles per hour, as jets do. Early jet engines were unreliable and would shut down unexpectedly. But that’s better than a “failure”. In airplane mechanic parlance, a jet engine failure is when the jet physically breaks apart. If the spinning pieces of metal are stopped by the engine’s body, the failure is contained. If the jagged-pieces-of-metal-that-were-recently-fan-blades rip through the engine’s collar and go flying-through-the-air-like-banshees, that is an “uncontained engine failure”. That phrase is printable, unlike the words I would exclaim were I ever in the vicinity of one.
So, good to know, but how does it change planes? Look at the wing of a B-29: http://www2s.biglobe.ne.jp/~hikouki/gallery/b29-wing.JPG [Ed. Note: I need to figure out how to embed images] The engines are physically in the wings. This is the easiest way to build wings, and why not? (Remember: these are giant car engines, and when you’re in a car you ride much closer to one.)
The engines on the B-47 are “podded”: http://en.wikipedia.org/wiki/File:B-36_engines,_Richie.jpg That is, they’re put in their own carries (called nacelles) and set down from the wing. Why? Two reasons, one of which drove the decision more than the other (which reason is which depends on who you ask).
First, it is easier to take an engine out of a pod than out of a wing. This makes it easier and cheaper to perform maintenance on podded engines.
Second, when uncontained engine failures happen, the shards fly through air, not through wing. Consider that the B-47’s customer was the Air Force, where uncontained engine failures are sometimes precipitated by surprising bullet interactions (is that suitably euphemistic), limiting failures was an engineering win.
Jet engines have gotten more reliable, requiring less maintenance and failures are extremely rare. But podded engines are still so good an idea that every modern jetliner uses podded engines.
So, good to know, but how does it change planes? Look at the wing of a B-29: http://www2s.biglobe.ne.jp/~hikouki/gallery/b29-wing.JPG [Ed. Note: I need to figure out how to embed images] The engines are physically in the wings. This is the easiest way to build wings, and why not? (Remember: these are giant car engines, and when you’re in a car you ride much closer to one.)
The engines on the B-47 are “podded”: http://en.wikipedia.org/wiki/File:B-36_engines,_Richie.jpg That is, they’re put in their own carries (called nacelles) and set down from the wing. Why? Two reasons, one of which drove the decision more than the other (which reason is which depends on who you ask).
First, it is easier to take an engine out of a pod than out of a wing. This makes it easier and cheaper to perform maintenance on podded engines.
Second, when uncontained engine failures happen, the shards fly through air, not through wing. Consider that the B-47’s customer was the Air Force, where uncontained engine failures are sometimes precipitated by surprising bullet interactions (is that suitably euphemistic), limiting failures was an engineering win.
Jet engines have gotten more reliable, requiring less maintenance and failures are extremely rare. But podded engines are still so good an idea that every modern jetliner uses podded engines.
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